Diamond is a preferred material for semiconductor devices because it has semiconductor properties that are superior to conventional semiconductor materials, such as silicon, germanium or gallium arsenide. Diamond provides a higher energy bandgap, a higher breakdown voltage, and a higher saturation velocity than these traditional semiconductor materials.
These properties of diamond yield a substantial increase in projected cutoff frequency and maximum operating voltage compared to devices fabricated using more conventional semiconductor materials. For example, silicon is typically not used at temperatures higher than about 200.degree. C. and gallium arsenide is not typically used above 300.degree. C. These temperature limitations are caused, in part, because of the relatively small energy band gaps for silicon (1.12 Ev at ambient temperature) and gallium arsenide (1.42 Ev at ambient temperature). Diamond, in contrast, has a large band gap of 5.47 Ev at ambient temperature, and is thermally stable up to about 1400.degree. C.
Diamond has the highest thermal conductivity of any solid at room temperature and exhibits good thermal conductivity over a wide temperature range. The high thermal conductivity of diamond may be advantageously used to remove waste heat from an integrated circuit, particularly as integration densities increase. In addition, diamond has a smaller neutron cross-section which reduces its degradation in radioactive environments, that is, diamond is a "radiation-hard" material.
Because of the advantages of diamond as a material for semiconductor devices, there is at present an interest in the growth and use of diamond for high temperature and radiation-hardened electronic devices. Diamond as a material for semiconductor devices may be either polycrystalline or single crystal. In particular, there is a present interest in the growth and use of single crystal diamond as a material for semiconductor devices. This interest is due in part to the increased efficiency of operation of single crystal semiconducting diamond in comparison with polycrystalline semiconducting diamond in which grain boundaries may impede the flow of charge carriers within the device.
Unfortunately, the fabrication of a single crystal diamond film is typically carried out by homoepitaxial deposition of a semiconducting diamond film on a single crystal diamond substrate. Such a single crystal diamond substrate is relatively expensive. In addition, large single crystal substrates may not be readily available for many desired applications.
A proposed microelectronic device having one or more semiconductor devices formed on a single crystal substrate, such as diamond, is described in U.S. Pat. No. 5,006,914 entitled Single Crystal Semiconductor Substrate Articles and Semiconductor Devices Comprising Same to Beetz. The patent discloses a microelectronic structure including a single crystal diamond substrate which is etched to form an array of spaced apart posts of single crystal diamond. On each post is grown a semiconducting layer of single crystal diamond to serve as an active channel region of a respective semiconductor device. Unfortunately, the use of a large single crystal diamond substrate as the starting point for the fabrication of the Beetz structure is relatively expensive.
Other attempts have been made to achieve some of the benefits of single crystal diamond while using less expensive and more readily fabricated polycrystalline diamond. For example, U.S. Pat. No. 5,036,373 to Yamazaki entitled Electric Device With Grains and an Insulating Layer discloses a light emitting device including a polycrystalline diamond structure wherein an insulating layer is provided between the diamond grain boundaries. The polycrystalline structure includes randomly positioned and unoriented grains. The insulating material prevents an overlying electrode from coming into electrical contact with graphite which is formed at the grain boundaries. Accordingly, a greater portion of the electrical energy may be converted into light rather than dissipated as heat energy.
Articles by Geis entitled Production of Large-Area Mosaic Diamond Films Approaching Single-Crystal Quality presented at the Electrochemical Society Meeting in Washington, D.C., May 6-10, 1991; and Device Quality Diamond Substrates appearing in Diamond and Related Materials, I, pp. 684-687 (1992), disclose a process for forming mosaic diamond films approaching single crystal quality. The films are formed by growing homoepitaxial diamond on an array of crystallographically oriented diamond seeds. The array of oriented diamond seeds are produced by passing a slurry of small (&lt;100 .mu.m diameter), faceted, diamond seeds over a substrate containing etch pits that match the diamond seed facets.
Unfortunately, the approach disclosed by Geis suffers from several shortcomings including a requirement that the diamond seeds must be mechanically positioned within each etch pit. Thus, voids will form where seeds are not positioned and good reproducibility of films is thereby degraded. The seed size must also be relatively large, and, accordingly, the minimum size of devices is typically at least 100 .mu.m. The quality of the films may also be degraded by contamination on the diamond seeds since such seeds are typically formed in a high temperature and high pressure process using a metal catalyst, such as nickel. The Geis process is also labor intensive and requires a relatively long growth time of up to several days for coalescence of the diamond film.
Various other techniques for forming diamond films for semiconductor applications have been proposed. For example, U.S. Pat. No. 4,915,977 to Okamoto et al. proposes forming a diamond film by evaporating carbon onto the substrate by arc discharge at a carbon cathode and applying a negative bias voltage to the substrate so as to form a plasma glow discharge around the substrate. U.S. Pat. No. 4,830,702 to Singh et al. proposes a hollow cathode plasma assisted method and apparatus for forming diamond films. Unfortunately, such electrical discharge methods for forming diamond films often fail to produce high quality diamond films.
Microwave plasma enhanced CVD has also been used to form diamond films. In addition, techniques have been developed for enhancing the nucleation of diamond onto a silicon substrate, or other substrate, for the subsequent growth of a diamond film by a conventional growth process. For example, a diamond nucleation density on a substrate may be increased several orders of magnitude by simply scratching or abrading the substrate prior to placing it into the conventional CVD growth chamber. Although the size and density of grown diamond particles can be controlled to some extent by the size and density of the scratches, each diamond particle still grows in a random orientation.
Other attempts have been made to more effectively seed the nucleation process, such as by spraying the substrate with diamond powder through an air brush, or by ultrasonically abrading the substrate surface. U.S. Pat. No. 4,925,701 to Jansen et al. proposes seeding a substrate with a diamond powder to enhance nucleation. Unfortunately, each of these types of preparation techniques has to be performed outside of the plasma CVD reaction chamber. The scratching and seeding techniques, also fail to produce a surface which is sufficiently smooth to permit in-situ monitoring of the diamond growth rate. Therefore, ex-situ analysis is commonly used, such as cross-sectional scanning electron microscopy or profilometry. Such ex-situ analysis does not permit processing parameters to be controlled during the diamond growth process.
An article entitled Generation of Diamond Nuclei by Electric Field in Plasma Chemical Vapor Deposition, by Yugo et al. appearing in Applied Physics Letters, 58 (10) pp. 1036-1038, Mar. 11, 1991, proposes a predeposition of diamond nuclei on a silicon mirror surface prior to a conventional diamond CVD growth process. A high methane fraction (i.e., at least 5 percent) in the feed gas is taught by Yugo along with a electrical bias of 70 volts negative with respect to ground applied to the substrate for a time period of just several minutes.
The Yugo article also proposes that a balance must be struck between the biasing voltage and the methane content of the gas mixture. The authors of the Yugo article theorize that an excessive acceleration of the ions from a high voltage can destroy newly formed diamond nuclei. Yugo suggests that revaporization of the newly formed diamond nuclei should be suppressed by mitigating the ion impact by keeping the magnitude of the bias voltage low. Thus, in order to offset the low bias voltage, the degree of carbon over saturation, as determined by the methane percentage, should be increased. Yugo reported that diamond nuclei growth did not occur below 5% methane content and that high densities of nuclei occurred only above 10% methane. In addition, the absolute value of the biasing voltages were maintained below 200 volts negative with respect to ground to avoid revaporization from high energy impacting ions. The total time duration for the pretreatment was limited to between 2 to 15 minutes.
The growth of heteroepitaxial, or textured diamond films comprising a plurality of locally heteroepitaxial diamond areas, therefore, is an important goal if the economical fabrication of diamond electronic devices, for example, is to become a reality. Heteroepitaxial or textured growth has been reported on cubic-boron nitride (c-BN), nickel and silicon. C-BN has shown promise as a heteroepitaxial substrate for diamond due to its close lattice match and high surface energy. However, it is presently difficult to grow c-BN in large single crystal form. Recent results report that local epitaxial growth of diamond on nickel is attractive. Nickel has a close lattice match with diamond although its catalytic properties on the decomposition of hydrocarbons into sp.sup.2 bonded structures may make it difficult to inhibit the formation of graphite during diamond growth and nucleation. Furthermore, it is difficult to obtain diamond films which adhere well to nickel.
An article by Jeng et al. in Applied Physics Letters, 56 (20) p. 1968, (1990), reported limited texturing of diamond on silicon substrates having a semicrystalline silicon carbide surface conversion film thereon. The lattice match between .beta.-SiC (a=4.36 .ANG.) and diamond (a=3.57 .ANG.) is not extremely attractive; however, .beta.-SiC grows epitaxially on Si despite a 24% lattice mismatch.
For many microelectronic applications there is a need for an array of interconnected semiconductor devices to provide higher power handling capability, for example. There is also a need to fabricate discrete devices economically, such as may be achieved by dicing a substrate containing an array of semiconductor devices, such as FET's. Moreover, there still exists a need to form high quality diamond films, such as textured diamond films approaching single crystal quality, to take advantage of the many attractive properties of diamond for semiconductor devices.